As represented by, for example, a plasma display panel (PDP) or a rare gas lamp to be used as a light source for reading out by a scanner, a VUV excitation light emitting element has been actively developed and practically used in recent years, which has a structure or a function such that a fluorescent layer employing a phosphor which emits light under excitation by VUV, is formed in an envelope made of e.g. glass, and a rare gas such as Ar, Xe, He or Ne alone or as mixed, is sealed in the envelope, so that the fluorescent layer in the envelope is excited by VUV radiated by discharge of the sealed rare gas, to emit light.
Heretofore, as a phosphor to be used as a fluorescent layer of such a VUV excitation light emitting element, a red emitting phosphor such as (Y,Gd)BO3:Eu, a green emitting phosphor such as LaPO4:Ce,Tb, Zn2SiO4:Mn, BaAl12O19:Mn, (Ba,Sr,Mg)O.aAl2O3:Mn or YBO3:Tb, or a blue emitting phosphor such as BaMgAl10O17:Eu or (Ba,Sr)MgAl10O17:Eu,Mn, has been used alone or as mixed.
As characteristics of the phosphor to be used as a fluorescent layer of a VUV excitation light emitting element, it is required that it emits light with higher luminance under excitation by VUV, that when a coating layer of the phosphor is subjected to baking treatment at a temperature of about 500° C. in a step of forming a fluorescent layer of a VUV excitation light emitting element, the emission luminance as the fluorescent layer will not decrease (luminance deterioration by baking is little), that even if the VUV excitation light emitting element is operated for a long period of time, and the phosphor is exposed to VUV continuously, the decrease in luminance of the phosphor (luminance deterioration by VUV) is little, and that the color purity of the emission color is good. However, phosphors which are presently practically used do not necessarily satisfy all of these characteristics. On the other hand, in the market, there is always a demand for more improvements of various properties of VUV excitation light emitting elements. Accordingly, also with respect to the phosphor for VUV excitation, it is desired to develop a new phosphor excellent in the above-mentioned characteristics.
Among phosphors for VUV excitation, an aluminate phosphor is a typical blue or bluish green emitting phosphor for VUV excitation, and a phosphor so-called BAM phosphor having bivalent Eu, or Eu and Mn, added as an activator to an aluminate of an alkaline earth metal containing Mg essentially as matrix crystal, such as BaMgAl10O17:Eu or (Ba,Sr)MgAl10O17:Eu,Mn, has been practically used as a blue or bluish green emitting phosphor for VUV excitation, excellent in the emission characteristics such as emission luminance. However, this BAM phosphor is a phosphor having drawbacks such that luminance deterioration by baking and luminance deterioration by VUV are substantial, and it is desired to develop a blue emitting or bluish green emitting phosphor for VUV excitation, whereby luminance deterioration by baking or luminance deterioration by VUV is little, which can be substituted for the BAM phosphor.
It is well known that an alkaline earth aluminate phosphor having Eu added as an activator to an alkaline earth metal salt containing no Mg in the matrix crystal, has the same aluminate as the BAM phosphor, exhibits blue emission when excited by ultraviolet rays having a wavelength longer than 200 nm (U.K. Patent 1,190,520, U.S. Pat. No. 4,827,187). Various properties as a phosphor for VUV excitation have not heretofore been known with respect to this phosphor, including a question as to whether or not this phosphor will emit light with high luminance under excitation by VUV.
The present invention has been made with an intention to develop a blue emitting novel phosphor for VUV excitation and has an object to provide an alkaline earth aluminate phosphor which has high emission efficiency and which exhibits blue color emission with excellent color purity with little luminance deterioration by VUV as a phosphor for VUV excitation, a phosphor paste composition employing such a phosphor, and a VUV excitation light emitting element.